Literature Review: Autism spectrum disorder (ASD) phenomenology is reported to be more common in some syndromes, compared to other syndromes. However, no statistical meta-analysis has yet been conducted, synthesising the prevalence data within and between syndromes. A literature search identified research reporting the prevalence of ASD phenomenology in 16 syndromes. Robust pooled prevalence estimates were generated for 12 syndromes. ASD phenomenology was evident in all syndromes and significantly more likely in all syndromes compared to the general population. Empirical Paper: The behavioural phenotype of Phelan-McDermid syndrome (PMS) is relatively unknown, but research has indicated atypically high levels of activity, impulsivity and ASD behaviours. The profile of ASD is also reported to be atypical. Carers of individuals with PMS (N=30; mean age=10.55, SD=7.08) completed questionnaires and these data were compared to data from matched samples with Fragile X and Down syndromes, and idiopathic ASD. The results revealed lower mood in individuals with PMS, but no difference in impulsivity and overactivity compared to the comparison groups. A total of 87% of individuals with PMS met criteria for ASD and 57% met criteria for autism. The profile of those who met clinical threshold for autism was homogenous, and analogous to those with idiopathic ASD.